1. Field of the Invention
This invention relates, generally, to the control of dc motors which are electronically, rather than mechanically, commutated and to the control of 2-phase, permanent magnet, brushless DC motors, in particular.
2. Prior Art
In motion control applications requiring high performance, incremental motion, the prime mover should possess highly linear torque characteristics versus applied current and a high resolution position feedback signal should be provided. Most applications require that these characteristics be met over a wide range of speeds with low ripple torque content, low audible noise, high power conversion efficiency and efficient thermal dissipation.
In the field of high performance, incremental motion control, the most commonly used motor technology is the dc servo motor with an integral encoder providing position feedback. The straight forward and well defined control criteria is the primary reason for the popularity of such motors. The motor is self-commutating, requiring that only a dc voltage (or current) be applied across a single pair of terminals to produce a torque to drive the load.
Unfortunately, this motor technology is relatively poor in reliability (due to brush/commutator wear), thermal characteristics (because it is rotor wound) and in terms of price/performance characteristics.
Alternative, but much less commonly used, motor technologies include the brushless DC (BLDC) and permanent magnet (PM) stepper type motors. Though different in construction, these two motor types are similar in that they contain permanent magnet rotors with wound stators and they must be electronically commutated. The range of applications of these types of motors has previously been limited by the difficulty in commutation while still requiring a high resolution position encoder to achieve a high level of performance. Also, they suffer from such undesirable characteristics as high torque ripple, high audible noise, as well as low and mid-range resonances. However, the thermal characteristics thereof are superior to the dc servo motor (because they are stator wound), the reliability is superior (because there are no brush or commutator segments to wear out) and the price/ performance characteristic is better. Moreover, electromagnetic interference is significantly reduced by the elimination of mechanical commutation.
BLDC motors are most commonly supplied in a 3-phase (Wye or Delta) configuration with Hall sensors imbedded in the motor to define commutation positions for each phase. The three phases are displaced 120 electrical degrees from each other. If the torque profiles of the individual phases are sinusoidal and the Hall sensors are ideally located, a ripple torque of about 14% of the output torque results. However, the Hall sensors are difficult to align, resulting in significantly more torque ripple thereby producing poor torque linearity, high audible noise and degraded power conversion efficiency. Thus, these motors are limited to high speed, low performance applications unless a high resolution encoder and fairly complex control algorithms are used.
Stepper motors are most commonly supplied in 2-phase configurations. (Four-phase configurations are also common. However, these are the same as 2-phase with different lead wire connections.) The phases in these motors are displaced by 90 electrical degrees from one another. They are commonly used in low performance, open-loop positioning applications. Control strategies include (in order of increasing performance) full, half and micro-stepping modes. Full and half-step modes are simple and inexpensive to implement, but suffer from high torque ripple, high audible noise, as well as low and mid-range resonances. Micro-stepping overcomes many of the undesirable characteristics, but at the expense of complexity and cost. However, in all of these operating modes the motor must be overdriven to insure reliable positioning; performance is dependent on constant or well behaved mechanical loads; and the torque-speed range is significantly less than that achievable under closed-loop control.
While the constructions and typical applications of the 2-phase BLDC and stepper motors differ vastly, the optimum control requirements therefor are strikingly similar. Simply stated, the two phases must be excited with sinusoidal currents which are in-phase with the respective torque profiles, where the resultant torque produced is linearly related to the magnitudes of the excitation currents. The two motor types complement each other well toward providing a wide range of torque/speed capabilities. The difference in the effective torque/speed ranges of the two is generally attributable to the electrical pitch (or commutation rate) of the motor phases.
Prior art technology in the field of closed loop control using induced EMF sensing is best represented by U.S. Pat. Nos. 4,275,343 and 4,455,513. However, the technology described in these patents (as well as other known technology) suffers from several performance limitations. For example, the known controllers do not maintain accurate position information in speed ranges down to, and including, zero velocity. The known systems also require a low-performance, open-loop, start-up sequence which is undesirable in many applications. Moreover, the prior art systems generally operate to commutate the motor with square waves of voltage or current which is quite limiting. Furthermore, the known systems do not accurately cancel variations in motor winding resistance, inductance and/or back EMF.
As a result, the best known technology currently in existence does not use the sensed position signals to achieve high performance position and velocity control in an effective, cost efficient manner.